1. Field of the Invention
This invention relates to a catalyst body used for an exhaust gas purification catalyst of an automobile engine.
2. Description of the Related Art
Various catalysts have been proposed in the past to purify detrimental substances emitted from an automobile engine. The exhaust gas purification catalyst generally uses a cordierite honeycomb structure having a high terminal impact resistance as a support. After a coating layer, formed of γ-alumina, is formed on a surface, a catalyst precious metal such as Pt is supported. The reason why the coating layer is formed is because the cordierite has a small specific surface area and cannot, as such, support the catalyst component. A necessary amount of the catalyst component can be supported when the surface area of the support is increased by use of a high specific surface material such as γ-alumina.
However, the formation of the coating layer invites an increase of a thermal capacity of the support and is therefore disadvantageous for early activation. Also, as the open area becomes small, a pressure loss will increase. Moreover, because γ-alumina has low heat resistance by itself, there is the problem that the catalyst component undergoes aggregation and purification performance greatly drops. Therefore, a method of directly supporting a necessary amount of the catalyst component without forming the coating layer has been sought in recent years. For example, Japanese Examined Patent Publication (Kokoku) No. 5-50338 proposes a method that conducts acid treatment and heat treatment to elute specific components and improves the specific surface area of the cordierite itself. However, this method involves the problem that the acid treatment and the heat treatment destroy the crystal lattice of the cordierite and lower the strength.
On the other hand, the inventors have previously proposed a ceramic support that does not require a coating layer for improving a specific surface area but can support a necessary amount of catalyst components without lowering strength (Japanese Unexamined Patent Publication No. 2001-310128). This ceramic support forms micro pores that cannot be measured as a specific surface area such as oxygen defect and lattice defect in a crystal lattice, very fine cracks having a width of 100 nm or below, and supports a catalyst. Therefore, the ceramic support can directly support the catalyst component while keeping the strength.
However, it has been found that, when the fine pores are formed on the surface of the substrate ceramic in accordance with the method described in Japanese Unexamined Patent Publication No. 2001-310128 and a catalyst body directly supporting a catalyst component is subjected to a thermal durability test (at 1,000° C. for 24 hours in an atmosphere, for example), the catalyst particle diameter becomes greater in some cases than before the thermal durability test. This is because the support strength varies depending on the kind of the substrate ceramic and depending on the combination of the fine pores and the catalyst component. Unless bonding with the catalyst component is sufficient, the catalyst component moves due to thermal vibration and the particles will grow, thereby lowering the purification performance.
It is therefore an object of the invention to acquire a catalyst body that exhibits less degradation of a catalyst due to thermal durability but can keep a higher catalyst performance for a long time.